Diode lasers are important in a wide variety of applications in optical communications, science, biophotonics, and many other areas. In an increasing number of applications, it is important to know the laser frequency with high precision. These applications include dense wavelength-division multiplexed (DWDM) optical communications, where the optical frequency must be controlled to typical 1-2 GHz. Systems using coherent optical detection require two laser with frequencies matched to at least sub-GHz accuracy. A variety of techniques for generating high-frequency microwave or terahertz radiation involve mixing together two wavelength-controlled lasers, and the resulting frequency is only as accurate as the control of the optical frequencies. For these applications and more it is important to be able to control the absolute wavelength of a diode laser source with a precision in the order of less than 10 MHz.
Most of the important applications discussed herein require single-frequency (distributed feedback (DFB) or distributed Bragg reflector (DBR)) lasers, but the methods disclosed are applicable to all types of diode lasers. Wavelength generally must be controlled actively, since the laser wavelength depends on device temperature and output power (which is proportional to the injection current), and this dependence changes as the laser ages. Active control is accomplished by measuring the laser wavelength and adjusting the laser temperature by means of a thermoelectric cooler (TEC).
A variety of methods exist to measure the wavelength of a laser diode. Wavelength meters operate by counting fringes in an optical interferometer. While highly precise, these instruments require mechanical parts to scan the interference pattern; hence they are too large and expensive for many applications. A scanning Fabry-Perot (FP) interferometer is a common device used for accurate frequency measurement. However, these are expensive, difficult to align, and require careful calibration with a known frequency source to provide absolute frequency measurement. Fiber FPs have been developed to offset cost issues somewhat, but these still lack absolute wavelength measurement. We seek a low-cost solution with absolute measurement capability.
It is well known that a wide variety of optical filters with fully characterized transfer functions (transmission or reflection versus wavelength) can be used to provide absolute frequency measurement. A variety of slope filters have been manufactured using, for example, thin film multilayer coatings to provide an approximately linear transfer function over a targeted range of wavelengths. The output signal then corresponds to a unique input wavelength. Since these can be made from materials (e.g., fused silica) with low thermal dependence, they can provide good thermal stability and temperature dependence that can be corrected easily. However, the accuracy of the wavelength control depends on the slope of the transfer function relative to the noise (noise from optical detection and from digital sampling) in the output signal. For wavelength tuning over a wide range, this slope becomes small, and the accuracy of wavelength control is impaired. We require the maximum possible accuracy; therefore this limited slope is unacceptable. In addition, a different filter is required for each operating range. This complicates supply and inventory management, as many different parts must be ordered, and this increases cost.
In optical communications systems using dense wavelength-division multiplexing (DWDM) wavelengths must be controlled to align the standard ITU frequency plan. Channels by this plan are defined on a periodic frequency grid with 100, 50, or 25 GHz frequency spacing. A typical DFB laser can be temperature tuned typically over several channels. To lock a laser to the center wavelength of an ITU channel, wavelength lockers have been developed. These generally consist of a Fabry-Perot etalon, made using thin-film multilayer dielectric minors. The mirror reflectivities are designed to produce resonances with a reasonably sinusoidal transfer function. While the exact peak of the FP resonances is angle and temperature dependent, the desired periodicity (100, 50, or 25 GHz) can be fabricated with high precision. Wavelength control is accomplished by locking the laser to the steep slope of the transfer function, where the transmission is typically in the middle between the maximum and the minimum. This provides a low-cost and accurate method for locking to discrete, specific, and periodic channels. However, the periodicity of the etalon transfer function causes ambiguity in wavelength measurement. Also, the measurement accuracy is decreased as the optical wavelength under measurement falls within the range around the maxima and minima (tops and bottoms) of the transfer function where the slope is decreased. Additionally, the accuracy at the bottoms is degraded more than that at the tops due to lower signal-to-noise ratio around the minima. Thus the use of a single etalon is not a suitable technique for precise wavelength determination over a wide wavelength range.
The measurement ambiguity due to the periodic transfer function of an optical etalon can be circumvented by adding a coarse wavelength determination method, as long as this method has a wavelength resolution no larger than the FSR of the etalon. One approach uses a linear spectral filter for the wavelength band of interest [U.S. Pat. No. 7,505,137 B2]. Alternatively the output wavelength of a semiconductor laser can be approximated by laser operation characteristics, including current, temperature and power.
Many approaches for improving measurement sensitivity of single etalon transfer function utilize a second etalon transfer function have been proposed. In one previous work, two high finesse etalons with different FSR are used to measure the wavelength of a tunable laser. But the absolute wavelength of the tuning range is measured by a wavelength meter and the tuning range is limited by the larger FSR between two etalons. Also, the method does not allow real-time wavelength monitoring [S. Yoshida, Y. Tada, and K. Nosu, “High resolution optical spectrum analysis by coherent detection with multi-electrode DBR-LD's as local oscillators,” in Conf. Proc. IMTC/94. IEEE Instrumentation Measurement Technology Conf., vol. 1, 1994, pp. 230-233].
Alternatively, the second periodic transfer function has same FSR as that of the first but with a phase shift of a fraction of FSR. Normally, two low finesse etalons with quadrature (90 degree) phase shift are used to mimic a pair of orthogonal sinusoidal response functions [U.S. Pat. No. 6,178,002 B1 and U.S. Pat. No. 7,420,686 B2]. In this method, the slope parts of one sinusoidal function cover the phase (optical wavelength or frequency) range where the other is at its maximum or minimum. The optical wavelength is then extracted from the pair of orthogonal signals. In U.S. Pat. No. 7,420,686 B2, the measurements at the maximum and minimum of the transfer functions (dead zones) are ignored. The finesse of two etalons has to be low (2 or less) to avoid dead zone overlapping and to make the etalon response relatively close to sinusoidal function. However, the very low finesse of an etalon gives a transfer function with a decreased contrast, which degrades the sensitivity due to decreased overall slope. Also, the quadrature phase shift is only optimal for a pair of etalons with this low finesse.
Furthermore, the binary method of deciding the validity of the measured signals in U.S. Pat. No. 7,420,686 B2 ignores the effect of the signal-to-noise ratio on the measurement accuracy. The accuracy is not only dependent on the slope of the etalon transfer function, but also sensitive to the noise in the signal output. The determination of the wavelength becomes less accurate when the input optical wavelength is close to the minimum of the transfer function due to both decreased slope and high relative noise. Therefore, generally, the measurement is more precise at a higher signal output (transmission or reflection) point than that of a low signal output even with a same function slope.
Accordingly, a method that optimizes the etalon finesse, relative phase shift, and wavelength algorithm based on detector sensitivity and signal-to-noise ratio so as to improve the performance of optical wavelength measurement and control is desired.